Electron Counting Model Research Articles

Reconstruction of the GaAs (311)Asurface

We study the reconstruction of the GaAs (311)A surface with reflection high-energy electron-diffraction (RHEED) and scanning tunneling microscopy (STM). The surface observed in situ with RHEED during molecular-beam epitaxy is distinguished by a lateral periodicity of 3.2 nm perpendicular to the [2\ifmmode\bar\else\textasciimacron\fi{}33] direction. This periodicity is confirmed employing in situ STM. High-resolution STM images furthermore reveal a surface reconstruction characterized by a dimerization of the surface As atoms. We show how the reconstruction can be formed, applying a simple electron-counting model. The excellent agreement with the experimental results further support this model, which was already found to explain the reconstructions of the (100) and the (111) surfaces. In order to form a semiconducting surface, the three uppermost layers are involved in the reconstruction process, giving rise to a depth modulation of 0.34 nm.

Ab initio study on the As-stabilized surface structure in AlAs molecular beam epitaxy

The dimer structure of an As-stabilized (001) AlAs surface was analyzed based on the cluster model by using ab initio molecular orbital calculations. The surface As dimer was modeled by Al4As4H12 (cluster 1) and Al7As8H19 (cluster 2) clusters. It was shown that the electron counting model was satisfied for cluster 2, while it was not satisfied for cluster 1. In both clusters, the electronic state of the reconstructed surface was shown to be differ greatly from that of the non-reconstructed surface. The ground state of cluster 2 was 1A1 , where a σ bond was formed between the two surface As atoms, and each surface As atom had a doubly occupied dangling bond. The bond length of the dimer was calculated to be 2.7 Å. The mechanism whereby the electron counting model was satisfied for cluster 2, but not for cluster 1, was clarified.

Ab initio study on the As-stabilized surface structure in AlAs molecular beam epitaxy

The dimer structure of an As-stabilized (001) AlAs surface was analyzed based on the cluster model by using ab initio molecular orbital calculations. The surface As dimer was modeled by Al 4As 4H 12 (cluster 1) and Al 7As 8H 19 (cluster 2) clusters. It was shown that the electron counting model was satisfied for cluster 2, while it was not satisfied for cluster 1. In both clusters, the electronic state of the reconstructed surface was shown to be differ greatly from that of the non-reconstructed surface. The ground state of cluster 2 was 1A 1 , where a σ bond was formed between the two surface As atoms, and each surface As atom had a doubly occupied dangling bond. The bond length of the dimer was calculated to be 2.7 Å. The mechanism whereby the electron counting model was satisfied for cluster 2, but not for cluster 1, was clarified.

Theoretical investigation of adsorption behavior during molecular beam epitaxy growth of GaAs: ab initio based microscopic calculation

This paper investigates, theoretically, the migration potential and adsorption energies of Ga adatoms on a reconstructed As-rich GaAs (001)-(2 × 4) surface by ab initio calculation. The calculated results show that migration potential depends sensitively on the number of surface Ga adatoms and that the adsorption energy oscillates with the adsorption of every other atom. The dependence on the number of surface Ga atoms is explained by the effects of the surface distortion and the electron counting model. This paper also discusses As incorporation during epitaxial growth and demonstrates the dynamical behavior of Ga adatoms at finite temperature by Monte-Carlo simulation.

Scanning tunneling microscopy of the GaAs (311)A surface reconstruction

Reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM) are used to study the reconstruction of the GaAs (311)A surface. During and after molecular-beam epitaxy, in-situ RHEED of the surface shows a lateral periodicity of 3.2 nm perpendicular to the [ 2 33] direction. This periodicity is confirmed employing in-situ STM. The height of this periodic surface structure is two monolayers, i.e., 0.34 nm. We show how this reconstruction, characterized by a dimerization of the surface As atoms, can be formed using a simple electron counting model. The excellent agreement with the experimental results further support this model, that was already found to explain the reconstructions of the (100) and (111) surfaces.

Scanning tunneling microscopy of the GaAs (311)A surface reconstruction

Reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM) are used to study the reconstruction of the GaAs (311)A surface. During and after molecular-beam epitaxy, in-situ RHEED of the surface shows a lateral periodicity of 3.2 nm perpendicular to the [ 2 33] direction. This periodicity is confirmed employing in-situ STM. The height of this periodic surface structure is two monolayers, i.e., 0.34 nm. We show how this reconstruction, characterized by a dimerization of the surface As atoms, can be formed using a simple electron counting model. The excellent agreement with the experimental results further support this model, that was already found to explain the reconstructions of the (100) and (111) surfaces.

Accounting for stoichiometry changes on compound semiconductor surfaces

We discuss how to self-consistently account for stoichiometry changes on surfaces with complex reconstructions, e.g. polar compound semiconductor surfaces. The key aspect of the methodology is that surface steps are allowed to act as a reservoir where atoms may be added or removed. The method is specifically applied to molecular beam epitaxy (MBE) and atomic layer epitaxy (ALE) results for GaAs(100) surfaces. The method easily demonstrates why only ∼ 12 monolayer (ML) of As is needed to convert from the Ga-rich surface to the (2 × 4) As-rich surface (as observed experimentally), even though the latter surface has an As coverage of 34 ML in the top layer. We also demonstrate how to convert from a more complex reconstruction, having two incomplete layers, to a simpler reconstruction having only one incomplete layer. The methodology also shows that ideal ALE of GaAs(100) cannot occur by cycling between the known adsorbate-free Ga-rich and As-rich surface reconstructions, because any such transition would not yield the observed 1 ML per cycle growth rate. We briefly discuss how adsorbates may stabilize ideally terminated (i.e. vacancy free) III–V surfaces. For example, methyl groups adsorbed on GaAs(100) exhibit a (1 × 2) LEED pattern, which is not seen for the clean GaAs(100) surface reconstructions. By using the electron counting model we interpret this structure as 12 ML CH3 adsorbed on a complete layer (1 ML) of dimerized Ga atoms. The ideal termination of the Ga-rich GaAs(100)-(1 × 2)-CH3 surface now allows for a plausible ALE mechanism which yields 1 ML deposition per cycle.

Surface stoichiometry and the role of adsorbates during GaAs atomic layer epitaxy

Several questions regarding the stoichiometry of atomic layer epitaxy (ALE) arise because most polar compound semiconductor surfaces reconstruct into structures terminated with less than monolayer coverage at the outermost layer. In this paper we briefly discuss how to self-consistently account for stoichiometry changes on surfaces that are not ideally terminated. The key aspect of the methodology is that surface steps are allowed to act as a reservoir where atoms may be added or removed. The methodology shows that ideal ALE of GaAs(100) cannot occur by cycling between the known adsorbate-free Ga-rich and As-rich surface reconstructions, because no such transition would yield the observed 1 monolayer (ML) per cycle growth rate. In fact, ideal ALE (1 ML/cycle) must involve at least one adsorbate induced surface reconstruction. Adsorbates may stabilize ideally terminated (i.e. vacancy free) III–V surfaces because of their ability to passivate dangling bond states. For example, methyl groups adsorbed on GaAs(100) exhibit a (1 x 2) LEED pattern, which is not seen for the clean GaAs(100) surface reconstructions. By using the electron counting model we interpret this structure as 1 2 ML CH 3 adsorbed on a complete layer (1 ML) of dimerized Ga atoms. The GaAs(100)-(1 x 2)-Ch 3 surface was also examined using surface infrared spectroscopy (SIRS) using a multiple internal reflection geometry. This surface exhibits relatively sharp infrared linewidths suggestive of a well ordered structure. The polarization dependence of the symmetric stretching and bending CH 3 modes also supports our proposed structure of the GaAs(100)-(1 x 2)-Ch 3 surface. The ideal termination of the Ga-rich GaAs(100)-(1 x 2)-CH 3 surface for a plausible ALE mechanism which yields 1 ML deposition per cycle.

Hydrogenation-induced energy gap of the As-terminated GaAs(100) surfaces.

First-principles total-energy calculations have been done for cases of monolayer-hydrogen adsorption on the As-terminated GaAs(100) surface. Two different geometries are considered as candidates for the stable geometry, one of which has been found to be metallic while the other has been found to be nonmetallic. The nonmetallic surface is more stable than the metallic surface despite the larger (smaller) number of dangling (chemical) bonds on the surface. The electronic properties and the order of the total energies of these two geometries are consistent with a prediction based on the electron-counting model, which is helpful to describe the atomic and electronic structures of pristine GaAs surfaces. We have concluded that the electron-counting model is also applicable in cases of hydrogen adsorption on GaAs surfaces.

Energetics of As dimers on GaAs(001) As-rich surfaces.

The stabilities of GaAs(001) As-rich surfaces have been determined for the first time by state-of-the-art total-energy calculations. The As-rich surface is found to be inherently rough; it cannot exist thermodynamically in a complete As monolayer structure, but in a missing-As-dimer or a two-As-layered structure. The steepening relaxation of the As-dimer block is essential to the stability of the As-rich surface. The peculiar interaction between As dimers as obtained, which may play an important role in the growth mechanism. The validity of the electron counting model is discussed.

First-principles calculations of molecular- and atomic-hydrogen reactions on As-terminated GaAs(100) surfaces.

We have investigated the reaction of molecular and atomic hydrogen on an As-terminated GaAs(100) surface, on which As dimers exist, by performing a first-principles total-energy calculation. According to the present calculations, the ${\mathrm{H}}_{2}$ molecule neither dissociates nor sits on the surface while the H atoms sit on several sites accompanied by As-dimer distortion. The As-dimer distortion is well described by the simple electron counting model, taking orbital energies into consideration. We have also found that the total energy of the H-atom-adsorbed surface is comparable to that of the GaAs(100) 2\ifmmode\times\else\texttimes\fi{}4 surface exposed to arsine and hydrogen gases.

Work function, electron affinity, and band bending at decapped GaAs(100) surfaces

Work function and band bending measurements are performed on n- and p-type GaAs(100) surfaces prepared by molecular-beam epitaxy and As decapping. The band bending is almost independent of the decapping temperature and surface reconstruction [(1×1)→C(4×4)→C(2×8)→(4×2)]. The work function and electron affinity undergo large and systematic variations of up to 400 meV which consistently establish maxima and minima of these quantities for the (2×4)-C(2×8) and (4×2)-C(8×2) structures, respectively. These variations are due to surface dipoles induced by charge exchanges between the cation and anion dangling bonds in the top two layers of the crystals. These charge exchanges are fully explained by applying a simple electron counting model to each reconstruction.

Electron counting model and its application to island structures on molecular-beam epitaxy grown GaAs(001) and ZnSe(001).

The principal reconstructions found on the low-index planes of GaAs and ZnSe can be explained in terms of a simple electron counting model. A surface structure satisfies this model if it is possible to have all the dangling bonds on the electropositive element (Ga or Zn) empty and the dangling bonds on the electronegative element (As or Se) full, given the number of available electrons. This condition will necessarily result in there being no net surface charge. The justification for this model is discussed. The GaAs(001)-(2\ifmmode\times\else\texttimes\fi{}4) reconstruction is known to involve surface dimers. It is shown that a (2\ifmmode\times\else\texttimes\fi{}4) unit cell with three dimers and one dimer vacancy is the smallest unit cell that satisfies the electron counting model for this surface. The electron counting model is used to explain the structure of islands imaged by scanning tunneling microscopy on the GaAs(001)-(2\ifmmode\times\else\texttimes\fi{}4) surface. The model shows that island structures built up from complete (2\ifmmode\times\else\texttimes\fi{}4) unit cells can be stable if they extend in the 2\ifmmode\times\else\texttimes\fi{} direction, but not if they extend in the 4\ifmmode\times\else\texttimes\fi{} direction. These island structures can also provide an explanation for the different step structures seen on GaAs(001) vicinal surfaces. Much less is known experimentally about step and island structures on ZnSe(001). Structures on this surface predicted by the electron counting model differ significantly from those found on GaAs(001).

Growth patterns in binary clusters of Group IV and V metals

Binary clusters composed of Group IV and Group V metals (e.g., Sn/Bi) are prepared by laser vaporization of corresponding alloys in a pulsed nozzle molecular beam source. Relative abundances of these clusters are studied with laser photoionization mass spectroscopy at 193 and 157 nm. Mass spectral abundance patterns for all systems studied (Sn/Bi, Pb/Sb, Sn/As) exhibit highly nonstatistical combinations of component elements. Preferential stoichiometries follow common patterns throughout these cluster systems, which can be related to the bonding and stability of condensed phase cluster ions of these same metals. These patterns are predicted by a simple valence electron counting model.